I'm gonna have to say it's the amount of nutrients your using or rather the amount of nutrients your not using. First I have to say that I am not familiar with superthrive and that you didn't list the amount that your using. I do have a lot of experience with GH 3-part, and your only using 2 parts. If you want big fat buds follow this link.

http://boards.cannabis.com/hydroponi...iscussion.html

It is a little confusing so if you have questions about it ask them in this thread